The present invention relates to a two-phase fluid cooling/heating circuit, commonly known as LHP (Loop Heat Pipe) circuit, and more specifically to a two-phase fluid cooling/heating circuit operating in a completely passive manner, i.e. without the aid of motor-driven/controlled components and/or electrical/electronic control systems.
LHP circuits are commonly used in particular in the aerospace field and in the aviation field (in particular military aviation) because of their characteristics of reliability, efficiency, reduced weight and low cost, but in particular because they are completely passive circuits and therefore do not require energy from an external source. As is known, an LHP circuit basically comprises an evaporator device with a first and a second portion which contain, as working fluid, a two-phase fluid and which communicate with each other via a porous wick. In the first portion, which acts as a reservoir or compensation chamber, the fluid is in the liquid phase, while in the second portion, which acts as the actual evaporator and which for this purpose is placed in contact with a body to be cooled (hereinafter referred to as “hot body”) so as to receive heat from this body, the fluid is in the vapour phase. The fluid moves by capillarity from the first to the second portion of the evaporator device through the porous baffle and then returns from the second portion back to the first portion flowing along a conduit and passing through a condenser device (made for example as a coil), where the transition from vapour phase to liquid phase takes place. The condenser device may be advantageously used also to release heat to a body to be heated (hereinafter referred to as “cold body”), and therefore the circuit is able to perform both the cooling function and the heating function, transferring heat through the two-phase fluid.
As already mentioned, the movement of the two-phase fluid along the circuit occurs as a result of the capillary thrust the fluid receives as it passes through the porous wick of the evaporator device. There is therefore no need for any pump or other device powered from the outside in order to ensure the flow of the fluid along the circuit, with evident advantages both in terms of manufacturing and operating costs, and in terms of reliability of the system.
Even though in the present description reference will be always made to a hot body and to a cold body, the circuit according to the invention may be equally well used to cool a hot fluid and heat a cold fluid. The terms “hot body” and “cold body” used in the description and in the claims of the present application are therefore to be understood as referring not only to solid bodies, but also to fluids.
EP 2 631 183 A1 discloses a temperature control circuit designed to control the temperatore of a heat source by varying the hydraulic resistance, that is to say, the pressure drop, in the circuit. For this purpose, the control circuit comprises a two-way control valve which controls the flow of the fluid from the evaporator to the condenser in response to the hydraulic resistance, i.e. the pressure drop, in the circuit, and which therefore is not a valve sensitive to the temperature of the two-phase fluid flowing in the circuit. This known control circuit does not comprise other control valves.
JP 2011 069546 A discloses an LHP circuit containing, inside a compensation chamber at the evaporator inlet, a valve which controls the flow of the fluid depending on the temperature in the compensation chamber. During normal operation the valve is closed and therefore causes the fluid to collect in the compensation chamber, while during the start-up phase it is open and therefore causes discharging of the fluid which has collected in the compensation chamber.
JP 2013 057439 A discloses an LHP circuit which, in order to eliminate the air bubbles upstream of the porous baffle to allow initial operation of the circuit, comprises a bellows valve designed to increase the pressure upstream of the porous wick. No further valves, in addition to the bellows valves, are provided for.
JP 2012 042115 A discloses an LHP circuit designed to cool electronic devices arranged in series. In order to allow bypassing of those electronic devices which temporarily do not dissipate heat and therefore do not need to be cooled, pairs of thermal expansion valves are provided for, which valves are designed to deviate the flow of the working fluid from the main circuit to a bypass branch.
WO 2008/050894 A discloses an LHP circuit for controlling the temperature of fuel cells comprising a thermal expansion valve associated with the condenser for controlling the flow of the fluid depending on the temperature.
The control circuits known from the prior art documents discussed above are not designed to keep the temperature of the working fluid (two-phase fluid) within a given range, in particular to keep the minimum temperature of the working fluid (temperature at the condenser) above a given minimum threshold value. Moreover, in order to disassemble the evaporator and the condenser, which are components which must be periodically inspected and cleaned (or replaced), these known control circuits require to empty the circuit of the working fluid contained therein, which results in longer and more expensive maintenance operations.